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To investigate the incidence, time to onset, and risk factors for the development of hypothyroidism after hemithyroidectomy.The incidence and risk factors for hypothyroidism in patients undergoing partial thyroid surgery remains unclear. Hypothyroidism is an under appreciated sequel of hemithyroidectomy. The early recognition of this postoperative complication may prevent symptoms of hypothyroidism and recurrent thyroid disease.We conducted a retrospective review of all patients undergoing hemithyroidectomy from August 1992 to June 2006 by a single surgeon. Patients were analyzed for age, sex, family history of thyroid disease, thyroid antibody levels, pre- and postoperative thyroid stimulating hormone (TSH), histologic diagnosis, weight of resected tissue, histologic evidence of thyroiditis, and lag time to diagnosis of hypothyroidism. χ2 or Fisher exact test or Student t test were performed.Hypothyroidism was diagnosed in 10.9% of 294 patients. The mean time to diagnosis was 8.2 ± 10.9 months. The mean thyroxine dose was 69.4 ± 31.6 micrograms. Patients with postoperative hypothyroidism had a higher incidence of a high-normal preoperative TSH level (TSH: 2.5–4.0 mIU/L, 18.8% vs. 3.8%; P < 0.01), thyroiditis on histology (46.8% vs. 11.8%; P < 0.01) and elevated thyroid antibodies levels (47.8% vs. 11.5%; P < 0.01). Age, gender, family history of thyroid disease, thyroid pathology, and weight of resected tissue were not significant risk factors for hypothyroidism.An incidence of 10.9% is higher than anticipated and all patients should have postoperative thyroid function assessment. However, a high-normal preoperative TSH, elevated thyroid antibody levels, and the presence of thyroiditis on histology are indications for close monitoring.